Isopentane prechilled to -150C is described as the best freezing method for what demonstration?

• A. Muscle enzyme demonstration
• B. Nerve conduction
• C. Lipid staining
• D. RNA integrity

Answer: (A)

Preserving enzymatic activity during tissue demonstration requires extremely rapid freezing to immobilize enzymes and keep their location intact. Isopentane prechilled to -150°C provides a very fast cooling rate and minimizes ice crystal formation, which can disrupt enzyme structure and diffusion. This makes it the best method for histochemical demonstrations of muscle enzymes, where you need enzymes to remain active and precisely localized so the staining accurately reflects their activity.

Nerve conduction is a functional measurement, not a histochemical stain, so this freezing method isn’t the focus there. Lipid staining can benefit from rapid freezing too, but preserving lipids isn’t the primary reason this particular method is favored for enzyme demonstrations. RNA integrity hinges on ultra-rapid freezing and strict RNase control, typically using liquid nitrogen to snap-freeze tissue; isopentane at -150°C isn’t the optimal choice for preserving RNA.